The invention relates of the transmission of a data-waveform-modulated signal utilizing an adaptive-delta-modulation transmission system.
When delta-modulating an analog message signal, for example a speech signal, the message signal is sampled at uniformly spaced instants in time, each sampled value is compared against the value of the delta-modulation approximation signal, and depending upon which of the two values being compared is the greater the modulator of the system assumes one or the other of its two states. The output pulse train of the modulator is then what is transmitted. The sampling frequency is so selected that, between two sampling instants, the amplitude of the analog message signal will not change by more than the amount of the quantization step. When the analog message signal is predictably of low amplitude, the magnitude of the quantization step should be made small, to keep low the quantization noise which is introduced during the process of message-signal reconstruction at the receiver. If the amplitude and frequency of the message signal is high, then the quantization step size should be great, to keep low the distortion which can result from slope-overload. Such distortion arises when the slope of the analog message signal exceeds the maximum effective slope which the approximation signal can achieve, i.e., exceeds the product of the quantization step size and the sampling frequency. Both forms of distortion, i.e., both quantization noise and slope-overloading, can be kept low if the size of the delta-modulation step is made variable. This can be done by automatically varying the step size in proportion to the slope of the analog message signal, the information concerning the slope of the message signal being derived not from the message signal itself but instead from the delta-modulated signal produced at the modulator output. Delta-modulation techniques utilizing varying step size are generally referred to as adaptive delta modulation.
When one utilizes adaptive delta modulation for the transmission of speech signals, it is customary to assume that, for the duration of a single spoken syllable, the amplitude of the envelope of the speech signal will stay approximately constant. Accordingly, within time intervals corresponding to the duration of a single spoken syllable, the amount by which step size is varied is conventionally to be kept small. For example, this viewpoint underlies the delta-modulation transmission system disclosed in German published patent application DT-OS No. 19 11 431. In that system, to ascertain the slope of the analog message signal, the step-size control circuit of the system examines, on an ongoing basis, three or more successive ones of the constituent binary pulses of the delta-modulated output waveform, to determine the extent to which these pulses are all of the same polarity. This sequence of determinations is averaged in an ongoing manner to generate a control signal used to automatically vary the step size.
In communications systems employing adaptive delta modulation, it may be necessary to transmit a message signal other than speech, for example a rectangular binary data waveform. When transmitting such data via telephone lines, for example, it is customary to utilize a modem technique. The rectangular binary data waveform to be transmitted is converted, using one form of modulation or another, into a data-waveform-modulated signal suitable for transmission along the transmission channel to be used. At the receiver end of the transmission channel, a corresponding form of demodulation is utilized, to recover the binary data waveform. Various modulation-demodulation techniques are customarily employed. For example, when bits are to be transmitted at a rate less than or equal to 1200 bits per second, it is customary to employ frequency-modulation. CCITT recommendation V23 makes a proposal for standardization in this connection, according to which the two characteristic frequencies 1300 Hz and 2100Hz are to constitute the symbols for the information to be transmitted. In contrast, when the data waveform to be transmitted has a bit stream frequency in excess of 1200 bits per second, it is now customary to utilize differential phase-shift keying (DPSK) for the modem. CCITT recommendations V 26, V 26 bis, V 27, V27 bis and V 27 ter provide proposals for standardization, here too. In differential phase-shift keying, most generally, the phase of a carrier signal is changed or reestablished (rekeyed) with a frequency f.sub.s corresponding to the frequency of the modulating symbol stream (e.g., with a frequency corresponding to the number of bits per second if the modulating symbol stream is a bit stream). The difference in the phase of the modulated carrier as between two successive rekeying instants represents the symbol to be transmitted. In an m-ary system, the number of different symbols which are to be transmitted (m=2 in a binary system) takes into account of course the utilizable bandwidth of the speech channel to be used and the transmission speed or bit stream frequency f.sub.b of the binary rectangular data waveform. In general, m=2.sup.n, wherein n is the number of constituent bits of a bit combination of the rectangular data waveform. Each possible binary state of such bit combination has associated with it a certain phase-difference value. The bit stream frequency f.sub.b is accordingly given by the equation f.sub.b =n.multidot.f.sub.s.
CCITT recommendation V 26 proposes a particular modem for a bit stream frequency of 2400 bits per second, utilizing four symbols (phase-difference values). The rectangular data waveform is formed into bit combinations of n=2, often called dibits. The symbol stream frequency (in this case often called the dibit frequency) accordingly amounts to 1200 Hz. If one is concerned with a bit frequency of 4800 bits per second and utilizes the modem proposed in CCITT recommendation V 27, the symbol frequency is 1600 symbols per second (n =3).
Another modulation technique used for modems is phase reference modulation. Here, the symbols to be transmitted are not represented by the difference in phase of the carrier at two successive rekeying instants, but instead by the phase of the carrier relative to the constant phase of a reference carrier.
These various modem techniques and others are used when using a rectangular data waveform (or more generally a data waveform which may or may not consist of rectangular pulses) to form a data-waveform-modulated signal which is to be transmitted by means of adaptive delta modulation. It is often necessary to be able to quantitatively characterize the transmission accuracy of such a modem plus adaptive-delta-modulation system. This is often done as follows. There is applied to the input of the delta modulation system not only the data-waveform-modulated signal to be delta-modulated transmitted but also a predetermined noise signal whose transmission level differs by a preselected amount from the transmission level of the data-waveform-modulated signal. One then compares the bits of the recovered rectangular data waveform at the receiver end against the bits of the original rectangular data waveform, to determine the bit error quotient. The transmission level of the data-waveform-modulated signal applied to the input of the delta modulator is then varied, in order to determine the functional dependence of the bit error quotient upon the transmission level of the data-waveform-modulated signal.
The difference between the transmission level of the data-waveform-modulated signal and the transmission level of the input noise signal is maintained constant, as the transmission level of the data-waveform-modulated input signal is varied. It proves to be the case that the bit error quotient is not constant. However, even more troublesome, the bit error quotient assumes a minimum value when the transmission level of the data-waveform-modulated input signal has a middle value, and as this transmission level is either increased or decreased relative to this middle value the bit error quotient increases rather steeply. This particular transmission-level-dependent behavior of the system is extremely troublesome; in practice, supposedly similar transmission lines may exhibit considerable differences in their attentuation characteristics, and one cannot with certainty so preselect the transmission level of the data-waveform-modulated input signal as to assure that one will avoid a region of high bit error quotient values.